1. Field of the Invention
This invention relates to single mode optical fibers and, more particularly, to reducing bend sensitivity and catastrophic bend loss in such fibers.
2. Discussion of the Related Art
In contrast with standard single mode optical fiber used, for example, in land line, undersea and metro systems, access fiber, which is typically located closer to the user, includes fiber-to the-home (FTTH), jumper cables, and FTTx fiber (e.g., fiber-to-the-curb, indoor wiring). Access fiber must not only interface in a low loss, reliable way with standard single mode fiber (SMF), which carries optical signals to the location being accessed (e.g., home, business, or other facility), but also must be relatively insensitive to the effects of bending, which is inherent in many of the access fiber applications.
Thus, in access fiber applications it is highly desirable to have fibers that combine low bend loss and good compatibility with existing infrastructure and standards. However, there is an inherent difficulty in achieving low bend loss without sacrificing properties important to compatibility, especially mode size, splice or connector loss, cutoff, and higher-order mode suppression. Ring-assisted or resonance-assisted fiber (RAF) designs alleviate these difficulties, but many previous RAF designs suffer from fabrication and bend range constraints. Fabrication constraints lead to higher cost and smaller preform size. In particular, the interior region (i.e., excluding the outer cladding) of a RAF has a refractive index profile fabricated using conventional vapor deposition techniques (e.g., MCVD). The various portions of the interior region (e.g., core, trench, ring/pedestal) have different refractive indices, which can be adjusted by doping with, for example, fluorine or creating hollow voids to produce a depressed-index region, or germanium to produce a raised-index region. Due to the large radial extent (cross-sectional area or volume) of the interior-region of a RAF compared to a conventional single mode fiber, a significant fraction of the fiber volume is deposited using the vapor-phase process. Since the deposition rate of such processes is relatively slow, this type of fiber material has relatively low throughput and hence relatively high cost.
Therefore, there is a need for a RAF design that allows at least a portion of the interior-region to be fabricated by a technique other than conventional, low-deposition-rate vapor deposition.
In addition to manufacturing cost, current RAFs exhibit an abrupt resonant coupling of fundamental mode signal light between the core and the ring, causing catastrophic optical loss at a critical bend radius, typically in the 3-5 mm range. Yet, recent industry studies have indicated that tight bend radii (2-4 mm) may occur in some installations and should be supported.
Therefore, there is also a need for a RAF design that alleviates the problem of catastrophic bend loss at a critical radius and provides low bend loss performance over a wider range of bend radii.